The present invention relates to a spray gun and in particular to a spray gun for use in spraying surface finishes and treatments. Particularly, but not exclusively, the invention is applicable to spray guns for the application of paint, and like material surface treatments, for example in the motor vehicle industry. The gun can be produced in three main forms, as a gravity fluid feed gun, a pressure fluid feed gun or an automatic oblique remotely-operated gun, all with single or multiple fluid feeds.
In the past, spray guns have used air input pressures of up to 620,000 Pa (ca. 90 psi) in order to achieve a head pressure (i.e. pressure at the air cap) of about 275,000 Pa (ca. 40 psi). High head pressure causes a cushion of air on the surface of the product being treated. This cushion forms a barrier that prevents the sprayed material reaching the surface and causes some of the sprayed material to bounce back and be displaced sideways by the following airflow and for it to be lost in the surrounding air.
Accordingly, this type of spray gun is very inefficient. Paint transfer efficiencies of greater than 35% are unusual, and the waste of paint material produces unacceptable emissions. of volatile organic compounds. In addition a solid residue can remain, this can be floating in the air for some time and may be highly toxic. These components are damaging to health. To overcome these problems it is necessary to reduce the air pressure and air volume used in such guns.
If the air pressure is reduced on a spray gun designed for high pressure use, the turbulence and restrictions in internal air passages, and in the air cap, cause a loss of air speed and a reduction in air volume. The result is low paint transfer rates, poor atomisation and an inferior paint finish, however transfer efficiency improves.
Existing high pressure spray guns have been modified to operate at low pressures, but the complexity of the designs and the intricate interconnecting drilled passages do not permit good air flow. In an effort to overcome the poor performance, increased air cap ring gaps are used, resulting in a substantial increase in air consumption. This type of spray gun has become known as the HVLP (high volume low pressure) type of spray gun.
More specifically, in prior spray guns and in the HVLP type of spray gun the means for actuating the valves which control and regulate the flow of liquid materials to be sprayed, and the pressurised air supply, and the interaction of these controls with the airflow passages to the spray nozzle, and the disposition of the nozzle relative to the remainder of the apparatus, leave considerable shortcomings. For example, it is commonplace that the stem of the needle valve with its associated compression spring and housing pass right across and through the path of the main air flow leading to a significant restriction in flow, air turbulence and energy loss. For example, as shown in WO 95/22409.
Likewise, in order to provide a convenient means for actuating the stem of the air flow valve and the fluid needle valve, the main spray nozzle of the apparatus is mounted on a forward projection of the apparatus so as to leave a free space to accommodate the arc of movement of the control valve trigger.
Moreover, where the same trigger operates both the liquid and air control valves directly, the progressive adjustment of the fluid control from on to off can influence operating characteristics of the air control valve which can be restricted in certain operating conditions. For example, this can occur when the liquid control valve has been manually adjusted to a point of nil flow which affects the ability of the trigger to operate both valves simultaneously through the full range of movement. Spray guns having a fluid flow restrictor valve or screw allow a full range of movement, however the control of fluid flow is no longer progressive.
The process of atomising fluid droplets in a spray gun is known to generate significant static electricity that becomes associated with the atomised spray droplets. Static charge on the droplets causes the spray to disperse and broaden due to repulsive forces. Thus, the work surface becomes progressively more charged during spraying. This causes strong electric field gradients to build up and repel incoming spray droplets, causing progressive reduction in transfer efficiency. Where charge persists during drying, airborne dust particles can be attracted onto the paint causing significant variations in the visual quality of the coated surface.
Static electrification during spraying is a problem which has persisted for many years in the painting industry. There have been several attempts to solve the problem but none have proved to be fully satisfactory. Attempted methods have involved providing earthed or conducting connections between the work and ground or between the work and the spray gun, or they have intentionally charged the work surface in an attempt to attract spray droplets (this is known as electrostatic spraying and is employed as a method of painting complex shapes without moving the spray equipment or work). While such devices may have significant beneficial effect on transfer efficiency, they will not apply static-free surface coatings.
Although some attempts have been made to incorporate a radioactive ioniser into the high pressure feed line of conventional high pressure spray guns in an attempt to neutralise static charges, due to poor design and unsuitable operating conditions,associated with high pressure spray guns and due to inappropriate design and positioning of conventional radioactive ionising cartridges, these attempts produced negligible benefits.
One object of the present invention is to provide apparatus for spraying a fluid such as paint or other surface treatment material, using a propellant, which may optionally be ionised, offering improvements in relation to one or more of the deficiencies of conventional spray guns described above.
Accordingly the present invention provides a spray gun having a cap with a spray nozzle; a propellant valve for controlling delivery of a propellant along a plurality of propellant passages to the spray nozzle; one or more fluid valves for controlling delivery of one or more fluids to the spray nozzle; and a common activation member provided on the central axis of the spray gun adapted to control both the propellant valve and the one or more fluid valves, wherein the plurality of propellant passages from the propellant inlet to the cap are substantially linear.
The propellant may optionally be ionised to reduce static and hence improve the efficiency of deposition of the fluid. Also, the spray gun may be adapted for either manual use incorporating a manually operable trigger, or may be suitably modified for use in automatic or robotic systems for example in-line systems in manufacturing plants.
The spray gun may optionally include an ioniser connected anywhere in the propellant passages downstream from the propellant inlet. With the spray gun of the present invention the propellant passages from the propellant valve to the nozzle may be short in length and substantially linear. In this way the ion density in the propellant flow may be maintained and the extent of ion losses through recombination at the walls of the propellant passages minimised.
The xe2x80x9cioniserxe2x80x9d is suitably chosen from a radioactive source, an X-ray ioniser or a high voltage corona discharge. For manual or hand-held spray guns the ioniser is ideally compact and lightweight to give the operator maximum convenience and ease of movement. For such manual spray guns the ioniser is preferably located within the handle of the spray gun, and the ioniser is preferably a radioactive source, and most preferably an alpha emitter especially the radioisotope polonium-210 (Po-210). The radioactive alpha emitter is preferably in the form of a sealed foil source. For automatic or robotic spray guns size and weight considerations are less important to the operation of the invention hence even quite bulky ionisers may be used, but where compactness is desirable radioactive sources are preferred.
The uni-axial design of the common activation member permits the use of a needle valve aligned along the central axis of the gun. Furthermore, one or more propellant passages are preferably located below a horizontal plane passing through the central axis of the gun. In this way the flow of propellant to the cap is unimpeded by the needle valve, unlike prior art designs. This provides an unrestricted, short path from the propellant inlet to the nozzle which minimises turbulence and energy loss and, where ionised propellant is being used, ionisation at the delivery point can be maximised.
Reference to the one or more passages being xe2x80x9csubstantially linearxe2x80x9d is intended as reference to the passages having a minimum number of deviations from linearity, i.e. bends in the propellant passages up to the cap. Preferably the number of deviations from linearity is zero or one, most preferably it is zero whereby impedance of the flow of propellant is minimised which in turn minimises turbulence of propellant flow to the cap. The propellant valve is designed and positioned so that it provides a substantially unrestricted, short, and substantially linear path for the propellant from the propellant inlet to the spray nozzle in the open position.
Thus, preferably, the ratio of inlet pressure to air cap pressure is less than 2, more preferably less than 1.5 and ideally 1.3.
In the fully open and in the partly open (transient) position the flow of propellant is guided by the shape of the valve housing into the one or more passages to minimise turbulence and energy loss. The propellant valve may consist of a piston and valve housing recess which are both tapered so that as soon as the valve mechanism is actuated the entire periphery of the propellant inlet is opened, even in the transient, partly open position, giving rise to progressive flow change without significant turbulence and energy loss.
The fluid flow control includes a needle adjustment housing which permits a progressive adjustment from zero flow to full flow without restricting the propellant flow.
Furthermore, the fluid inlet may be located immediately adjacent to the spray nozzle so that viscous fluids such as high solids paint may be sprayed using the gun.
In a further aspect the present invention provides a spray gun having a spray nozzle and a propellant valve at a propellant inlet for controlling delivery of propellant along one or more passages to the spray nozzle wherein an ioniser for ionising propellant passing through the ioniser is provided adjacent the propellant inlet.